<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
   <style>
       #parent {
           width: 100%;
           height: 500px;
           margin: 0 auto;
           display: table;
       }
       .row {
           display: table-row;
       }
       .item {
           border: 1px solid #000;
           display: table-cell;
           height: 200px;
       }

   </style>
</head>
<body>
<div id="parent">
    <div class="row">
        <div class="item">1</div>
        <div class="item">2</div>
    </div>
    <div class="row">
        <div class="item">4</div>
        <div class="item">5</div>
    </div>
    <div class="row">
        <div class="item">7</div>
        <!--<div class="item">8</div>-->
        <!--<div class="item">9</div>-->
    </div>
</div>

</body>
</html>